Aluminum wire cables are increasingly being incorporated in automotive wiring harnesses. There are several factors driving this trend. First, the market price of copper, traditionally used for automotive wiring, is significantly higher and more volatile than that of aluminum. Second, aluminum wiring is approximately half the weight of the equivalent copper wiring. The push for more fuel efficient vehicles is requiring weight to be taken out of the vehicle and aluminum-based wire cables provide a good opportunity to reduce weight by substituting aluminum cable for copper cable.
Aluminum does have some disadvantages for vehicle wiring applications. Aluminum cable typically has multiple strands in the wire cable to increase flexibility of the cable. Aluminum forms an oxide layer that has a lower electrical conductivity than aluminum and is very hard. Because these aluminum oxides form almost instantaneously on the surface of the aluminum wires when exposed to air, individual wire strands in the cable do not electrically connect well to each other. Additionally, aluminum wire strands in a crimped connection can also deform over time due to stress relaxation and creep. As the aluminum wire in a termination changes shape, electrical resistance can increase causing increased connection resistance, heat build-up, and connector failure. Because of these challenges, conventional crimped connections to aluminum cable constructions with a large number of strands do not provide a robust low resistance connection that will withstand environmental exposure over time. Alternate connection technologies are required to help interconnect each strand in the cable core so they act as a single conductor. It may also be desirable to use existing wire cable terminals, hereafter referred to as terminals, designed for copper cable connection systems to avoid the expense of designing new terminals made especially for aluminum cable.
One method of interconnecting the wire stands is ultrasonically welding the wire stands together. In some cases, the wire stands are directly ultrasonically welded to the terminal. This process, however, requires a terminal with a wire connecting end that is designed specifically for ultrasonic connection. These ultrasonic terminals require different tooling than the crimp connectors typically used for copper wire cables. Alternatively, the wire strands may be ultrasonically welded together forming a generally rectangular cuboid “nugget”. Examples of methods and apparatus that may be used to ultrasonically weld the ends of wire cables to form a rectangular cuboid nugget are found in U.S. Pat. No. 4,730,764 issued to Hawkins et al. on Mar. 15, 1988 and U.S. Pat. No. 4,867,370 issued to Welter et al. on Sep. 19, 1989. After welding, the nugget may be trimmed to a desired length and dipped in a liquid sealant prior to crimping the terminal. An example of such a method of applying a sealant to a crimped wire connection may be found in U.S. Pat. No. 8,181,343 granted to Martauz, et al. on May 22, 2012. The coated nugget is then attached to the terminal by crimping the nugget to the terminal. The process of crimping the terminal to the rectangular weld nugget creates stresses in the nugget which may cause the nugget to fracture. In some cases, this nugget fracture results in voids in the sealant coating. Therefore, it is desirable to form a nugget that interconnects the wire strand that is resistant to fracture when crimped to a terminal.
FIG. 1 illustrates a wire cable 12 attached to a wire cable terminal 14 according to the prior art. The uninsulated individual stands 16 of an end of the wire cable 12 are joined together to form a solid shape 18 that generally defines a rectangular cuboid or rectangular prism. The formed shape 18 of the end of wire cable 12 is hereafter referred to as a “nugget” 18. The nugget 18 may be formed by ultrasonically welding the wire strands 16 together. This is especially beneficial when aluminum wire cables are used because the ultrasonic welding breaks down surface oxides on the wire strands 16 of the aluminum cable, improving strand-to-strand conductivity. The wire cable 12 is then attached to the terminal 14 by placing the nugget 18 into the base portion 20 of the terminal 14 and crimping the wire crimping wings 22 over the nugget 18 and the insulation crimping wings 24 to the wire cable insulation 26. A liquid sealant (not shown) may be applied to the nugget 18 prior to attaching the terminal 14 to seal the surface of the nugget 18 and penetrate inter-strand voids to inhibit galvanic corrosion that may occur if the wire cable 12 and the terminal 14 are formed of dissimilar metals, such as an aluminum wire cable and a copper-based terminal.
As illustrated in FIG. 2, the nugget 18 formed by the prior art method may fracture due to stress induced by the wire crimping wings 22 after the wire crimping wings 22 are crimped to the nugget 18. The fracture may form a void 28 in the nugget 18 that is unprotected by the applied sealant. This void 28 may present a site that is subject to galvanic corrosion.
The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions.